1. Field of the Invention
This invention relates to compositions which exhibit the electro-rheological (ER) effect. More particularly, this invention relates to electro-rheological (ER) compositions, also sometimes termed electro-viscous (EV) compositions, which contain no water, can be operated stably even in an increased temperature, and can be used for power transmission devices and damping equipment such as, for example, printers, valves, clutches, dampers, shock absorbers, vibrators, engine mounts, and actuators.
2. Description of the Prior Art
The ER effect is such a phenomenon that when dielectric substances are dispersed in an electrical insulating medium, the viscosity thereof increases remarkably under the influence of an electric field applied thereto due to the orientation of these substances. As the electrical insulating medium, silicone oil, fluorinated silicone oil (JP-6-192672,A, for example), transformer oil and the like are used. On the other hand, as the dielectric substance, silica, barium titanate, ion-exchange resins, argillaceous minerals (JP-7-258412,A), starch, metal, etc. are used.
Further, it is known in the art that when the dielectric substance contains a small amount of water, the ER effect will be improved remarkably (Development of Electro-rheological (ER) Fluid, CMC, 1999).
When the dielectric substance contains water, however, the usable temperature range is restricted to the range in which water can maintain its liquid state and the performance of the ER fluid is extremely deteriorated in a lower temperature range and a higher temperature range. Further, the addition of water will enhance the electrical conductivity of the whole of the system and permits the passage of an electric current, which poses such a drawback that a power supply to be needed becomes large. Moreover, the system generates heat by the electric current and the heat generation is runaway. As a result, the deterioration of its performance is promoted.
For the purpose of eliminating such drawbacks, JP-5-17791,A proposes to use solid particles comprising conductor or semiconductor oxide particles each containing an electrical insulating oxide layer formed on the surface thereof. When the system to be used contains as the dielectric substance such conductor or semiconductor oxide particles covered with an inorganic oxide layer, the usable temperature range becomes widened. However, it poses the problem of not being suitable for long-term use because fine particles generated by the mutual collision of particles and wear thereof while in use are dispersed in the whole system and, as a result, the insulating ability of the whole system will be decreased to such a degree that the stability is deteriorated.